1. Field of the Invention
This invention relates to the field of flexural bearings, and in particular to multiple degrees of freedom of movement flexural thrust joints for precision stages.
2. Description of the Related Art
Precision stages find many applications with steadily increasing precision requirements. In semiconductor capital equipment, for example, precision stages are required for carrying wafers during lithography process steps. As the length scales of microcircuit features become smaller, modern lithography moves toward the use of higher resolution techniques, for example phase shift masks, extreme ultraviolet (EUV) systems, and high throughput charged particle beam lithography. With the overall trend toward smaller features, positioning requirements of the wafer (and a stage platform upon which it rests) relative to lithography optics have become increasingly stringent.
Due to the comparative magnitudes of charged particle wavelengths and sub-micron features, charged particle (especially electron) beam lithography is an important technique for realizing sub-micron feature sizes. However, in charged particle beam lithography systems, vacuum and high voltage compatibility is an inherent design complication.
In order to accelerate a charged particle beam, for example an electron beam, particle beam lithography systems typically have electric potential differences (up to 100 kV) between different components. In many designs, a workpiece or wafer is held at a potential close to electrical ground and the electron source is at comparatively higher voltage. However, in some systems, the electron source is operated at a potential close to electrical ground, while the workpiece is held at comparatively higher voltage. Such an electron source is described in U.S. Pat. No. 5,637,951.
A vacuum is also required within the lithography device to allow the propagation of an electron beam. A high vacuum, with pressure of no more than 10xe2x88x926 Torr, is typically required. Thus, in addition to providing the required precision, a stage for electron beam lithography must be able to sustain high voltage differences between components and be suitable for operation under high vacuum conditions.
In regard to high voltage operability, many prior art precision stages comprise several closely coupled kinematic platforms. An exemplary prior art device of this type is shown in FIG. 1. However, in a system where up to 100 kV potential difference must be held off with no arcing or other undesirable effects, close mechanical coupling of non-insulating components, such as illustrated in FIG. 1, would be inappropriate.
In regard to vacuum compatibility, many prior art stages are inappropriate for use in a high vacuum due to bearing designs that are prone to out-gassing, generating particulates, or otherwise contaminating the vacuum system. In comparison to other bearing types, flexural bearings are very well-suited for use in vacuum, and allow smooth, precise, predictable motion of the stage. Such properties are discussed in detail by Alexander Slocum, xe2x80x9cPrecision Machine Designxe2x80x9d, Prentice Hall, N.J., 1992. However, most prior art flexural bearings are potentially unstable when used as thrust bearings since the flexural joint is prone to distortion or buckling under compressive loads. Thus, in precision stage applications requiring vacuum compatibility, there is a need for multiple degrees of freedom of movement flexural thrust joints.
Further, for positioning systems requiring precision movement and short times to reach a steady-state position (settling times) after each movement, attention must be paid to perturbations that are insignificant for less precise systems. In order to optimize the mechanical stability of the stage under external impulses, vibrations or other mechanical perturbations, most prior art high precision lithography stages utilize massive, high inertia elements. However, while a high inertia system tends to be mechanically stable, the reaction forces associated with moving a massive stage can degrade the accuracy and precision of the relative positioning of the lithography optics and the work platform, unless a long settling time is accepted. A combination of careful isolation of the optics and addition of considerable mass to the optics mounts is required to overcome this problem. The resulting weight and volume of such devices is undesirable. It would be beneficial, then, to have a comparatively low-mass high precision stage that avoids these problems. Furthermore, such a low-mass stage would be well-suited for commercial lithography tools with their high throughput requirements, where frequent large accelerations and decelerations are required for rapid processing of the workpiece.
Therefore, especially for use in charged particle lithography systems, there is a need for stages capable of maintaining high precision alignment with a reference (for example the position of the lithography optics) while being vacuum compatible and capable of sustaining large voltage differences between elements. Such platforms should be continuously and smoothly movable over six degrees of freedom of movement. Such platforms should also be capable of holding workpieces with characteristic lengths of hundreds of millimeters, e.g. 300 mm silicon wafers.
This invention includes a flexural joint, in particular a flexural thrust joint. The joint may be used in applications requiring vacuum, and is well suited for use in precision stages for charged particle lithography and more particularly for multicolumn charged particle lithography. According to aspects of the invention, the flexural joint comprises: a first joint element, the first joint element having a first load bearing surface, the perpendicular to the first load bearing surface defining a first direction; a second joint element, the second joint element having a second load bearing surface, the perpendicular to the second load bearing surface defining a second direction, the second direction being opposite to the first direction; and two first flexure strips attached to the first and second elements at first and second flexure strip attachment surfaces, respectively, wherein the first and second flexure strip attachment surfaces are separated along the first direction, in respective order, whereby the first flexures are under tension when the joint is subject to a compressive load. The bending of the flexures (an intrinsic property) gives the joint one degree of freedom of motion. When the flexures are sufficiently long and thin, they are able to twist, as well as bend; this provides the joint with two degrees of freedom of motion.
In a further embodiment, the joint comprises: a first joint element, the first joint element having a first load bearing surface, the perpendicular to the first load bearing surface defining a first direction; a second joint element, the second joint element having a second load bearing surface, the perpendicular to the second load bearing surface defining a second direction, the second direction being opposite to the first direction; a third joint element; two first flexure strips attached to the second and third elements at second flexure strip attachment surfaces and upper third flexure strip attachment surfaces, respectively, wherein the second and upper third flexure strip attachment surfaces are separated along the second direction, in respective order; and two second flexure strips attached to the third and first elements at lower third and first flexure strip attachment surfaces, respectively, wherein the lower third and first flexure strip attachment surfaces are separated along the second direction, in respective order, whereby the first and second flexures are under tension when the joint is subject to a compressive load. When one or both sets of first and second flexures are sufficiently long and thin to twist, the joint has three degrees of freedom of movement.